Abstract
Polyacrylonitrile-based pre-oxidized fibers with improved thermal stability, flame retardant, and mechanical properties were made from the pristine polyacrylonitrile fibers through chemical pretreatment followed by pre-oxidation in air. The morphological structure of the polyacrylonitrile-based pre-oxidized fibers was investigated by Fourier transfer infrared spectra, X-ray diffraction, scanning electron microscopy, and X-ray energy dispersive spectrometer. The changes of characteristic functional groups and chemical compositions confirmed the successful modification of the polyacrylonitrile fibers during pre-treatment. The grooves and cracks on the surface of polyacrylonitrile-based pre-oxidized fibers were remarkably decreased in comparison with that of pristine polyacrylonitrile fibers. The evolution of crystalline structure of the polyacrylonitrile fibers proved the occurrence of cyclization reactions during pre-oxidation. Meanwhile, thermal stability, flame retardant, and mechanical properties of polyacrylonitrile-based pre-oxidized fibers were also investigated by thermogravimetric analyzer, oxygen index meter, micro combustion calorimeter, and single fiber tensile tester, respectively. The results demonstrated that the polyacrylonitrile-based pre-oxidized fibers initially pre-treated by hydroxylamine hydrochloride, followed by monoethanolamine, had a high limiting oxygen index of 40.1 and breaking strength of 2.03 cN/dtex. The peak of heat release rate and total heat release of polyacrylonitrile-based pre-oxidized fibers decreased significantly while its charred residues increased, contributing to the improved flame retardant property.
Highlights
The bands centered at 2243 cm−1 and 1732 cm−1 were attributed to the stretching vibrations of cyano (C≡N) and ester (C=O) groups, respectively
After chemical pre-treatment, the intensity of characteristic peak of the C≡N groups at 2243 cm−1 was significantly decreased and the occurrences of new absorption bands located at about 1647 and 911 cm−1 were assigned to the stretching vibrations of C=N and N–O groups
Microscale combustion calorimetry (MCC) was employed to measure the combustion property of oxidized polyacrylonitrile fibers (OPF), and the relevant parameters including the peak of heat release rate (PHRR) and total heat released (THR), which were important indicators to reflect combustion characteristics and evaluate fire safety.[30,31]
Summary
At the same pre-oxidation conditions, the degree of cyclization of the OPF through chemical pretreatment with HA followed by MEA (i.e. OPFHA-MEA) was higher, which would inevitably affect the flame retardant and mechanical properties of OPF. It was speculated that the chemical pre-treatment combined with HA and MEA might introduce more amide groups into the PAN macromolecular side chains, which promoted the cyclization reactions and formed a stable cyclic pyridine structures.
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